(a) Technical Field
The present invention relates to a humidification system for a fuel cell. More particularly, the present invention relates to a humidification system with membranes of different species, in which a material having a high humidification performance and capable of being swollen with water is arranged in the center of a hollow fiber membrane bundle disposed in a hollow fiber membrane module and a material that is not swollen with water is disposed on the outside thereof.
(b) Background Art
It can be necessary to humidify an electrolyte membrane in a fuel cell for the operation of the fuel cell, and thus a humidification system, in which water of exhaust gas, i.e., wet gas, discharged from the fuel cell is added to dry air supplied from the outside, is used.
Fuel cells can require a compact humidification system that occupies little space for mounting, and requires low power consumption. Humidification systems designed for satisfying such demands include various techniques such as ultrasonic humidification, steam humidification, and evaporative humidification, etc.; however, a humidification technique using a hollow fiber membrane is suitably used for the fuel cell.
FIG. 1 is a schematic diagram illustrating an air supply system of a fuel cell system.
As shown in FIG. 1, the air supply system of the fuel cell system includes a membrane humidifier 100, to which dry air is supplied from the outside by a blower 202 and through which exhaust gas discharged from a fuel cell stack 200 passes. Accordingly, the dry air supplied from the outside is humidified while the exhaust gas containing water passes through hollow fiber membranes.
FIG. 2 is a cross-sectional view showing a schematic configuration of a hollow fiber membrane humidifier.
As shown in FIG. 2, the humidifier 100 includes a housing 101 having a first inlet 102 for introducing dry air and a first outlet 103 for discharging dry air. Moreover, a hollow fiber membrane module 107 is provided in the housing 101, and a plurality of hollow fiber membranes 106 is placed in the hollow fiber membrane module 107.
The operation of the hollow fiber membrane humidifier 100 having the above configuration will be described below.
When exhaust gas, i.e., wet air, discharged from the fuel cell stack is supplied to the inside of the hollow fiber membrane module 107 through a second inlet 104 of the housing 101, water in the wet air is separated by capillary action of the respective hollow fiber membranes 106, and the separated water is condensed while passing through the capillaries of the hollow fiber membranes 106 and moved inside the hollow fiber membranes 106.
Subsequently, the air from which water is separated is transferred to the outside of the hollow fiber membranes 106 and discharged through a second outlet 105 of the housing 101.
Meanwhile, the outside air (dry air) is supplied through the first inlet 102 of the housing 101 by the operation of the blower and moved along the inside of the hollow fiber membranes 106. At this time, since the water separated from the wet air has been transferred to the inside of the hollow fiber membranes 106, the dry air is humidified by the water, and the thus humidified air is discharged to the fuel cell stack through the first outlet 103.
However, as shown in FIG. 2, since the hollow fiber membrane module 107 has a structure in which the plurality of hollow fiber membranes 106 is densely packed, it is difficult for the wet air introduced through the second inlet 104 to penetrate into the inside of the hollow fiber membrane module 107.
Moreover, since the wet air is diffused very slowly, the difficulty becomes more considerable.
For such reasons, in the hollow fiber membrane module 107 accommodated in the housing 101, the wet air passing through the outside of the hollow fiber membrane module 107 does not penetrate into the center of the hollow fiber membrane module 107 shown as a dotted line box in FIG. 2, but mainly flows along the edge of the hollow fiber membrane module 107 as shown with arrows in FIG. 2. As a result, the rate that the wet air penetrates into the inside of the hollow fiber membrane module 107 is very low, and thus the humidification efficiency is reduced.
Accordingly, the hollow fiber membranes 106 positioned in the vicinity of the center of the hollow fiber membrane module 107 are not supplied with sufficient water, and thus the overall efficiency of the humidification system may be reduced.
Moreover, in case of the conventional humidifier 100, since the dry air introduced through the first inlet 102 mainly flows through the center of the hollow fiber membrane module 107 (the portion shown as a dotted line box in FIG. 2), the overall humidification efficiency may be greatly reduced.
Such a problem is illustrated in the simulation test results of FIG. 3.
It can be clearly seen from FIG. 3 that most of the dry air flows only through the center of the hollow fiber membrane module 107.
In other words, since the dry air introduced through the first inlet 102 mainly flows through the center of the hollow fiber membrane module 107 (the portion shown as a dotted line box in FIG. 2) and the wet air introduced through the second inlet 104 flows along the edge of the hollow fiber membrane module 107, the overall humidification efficiency may be reduced.
Such a problem becomes more serious when the amount of dry air is increased, i.e., when the fuel cell stack provides a high output voltage.
Another problem of the conventional humidification system is caused by the hollow fiber membranes and the arrangement thereof.
Despite the advantage that the membrane humidifier is applicable for use with a vehicle, available hollow fiber membrane materials are very expensive, and thus it disadvantageous especially in terms of the manufacturing cost.
In the case of membrane humidifiers, a Nafion membrane is widely used as the hollow fiber membrane material applied to the fuel cell stack, more particularly, to a membrane electrode assembly (MEA), and thus it is disadvantageous in terms of cost reduction.
In most cases, sufficient humidification is required in a low current region of the fuel cell system, and much water is produced in high power and high current regions to the extent that a cathode does not require humidification. Nevertheless, most of the humidification systems known at present are operated without varying the amount of humidification in both the low and high current regions. Especially, in the case where only Nafion is used as the material for the hollow fiber membranes, a high humidity of more than 80% RH [what does the abbreviation RH stand for?] is provided even in the high current region.
Since a large amount of water is produced and a high humidity is provided in the high current region of the fuel cell system, an increase in resistance of cathode material transfer and a flooding phenomenon may occur, which results in air starvation of the cathode. As a result, the deterioration of the fuel cell catalyst is accelerated and thus the durability of the fuel cell is reduced.
Moreover, as shown in FIG. 4, in the case where the whole bundle of hollow fiber membranes 106, disposed in the hollow fiber membrane module 107 of the humidification system, is formed of NAFION, the bundle of hollow fiber membranes expands by absorbing water due to its characteristics, and the hollow fiber membrane itself is not stretched in the longitudinal direction thereof but rather bent in a zigzag shape due to the expansion, thus increasing pressure drop in the humidification system. As a result, the load applied on the air blower for supplying air to the humidification system is increased.
Similarly, in the case where only Nafion is used as the material for the hollow fiber membranes, there are numerous problems to be overcome.
The above information disclosed in the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.